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Processing of Plastics 327

(a) pretorm in position (b) Melerial forced into cavities
Fig. 4.64 Transfer moulding of thermosetting materials
4.9 Processing Reinforced Thermoplastics

Fibre reinforced thermoplastics can be processed using most of the conven-
tional thermoplastic processing methods described earlier. Extrusion, rotational
moulding, blow moulding and thermoforming of short fibre reinforced ther-
moplastics are all possible, but the most important commercial technique is
injection moulding. In most respects this process is similar to the moulding of
un-reinforced thermoplastics but there are a number of important differences.
For example the melt viscosity of a reinforced plastic is generally higher than
the unreinforced material. As a result the injection pressures need to be higher,
by up to 80% in some cases. In addition the cycle times are generally lower
because the greater stiffness of the material allows it to be ejected from the
mould at a higher temperature than normal. However, the increased stiffness
can also hamper ejection from the mould so it is important to have adequate
taper on side walls of the cavity and a sufficient number of strategically placed
ejector pins. Where possible a reciprocating screw machine is preferred to a
plunger machine because of the better mixing, homogenisation, metering and
temperature control of the melt. However, particular attention needs to be paid
to such things as screw speed and back pressure because these will tend to
break up the fibres and thus affect the mechanical properties of the mouldings.
A practical difficulty which arises during injection moulding of reinforced
plastics is the increased wear of the moulding machine and mould due to the
abrasive nature of the fibres. However, if hardened tool steels are used in the
manufacture of screws, barrels and mould cavities then the problem may be
negligible.
An inherent problem with all of the above moulding methods is that they
must, by their nature, use short fibres (typically 0.2-0.4 mm long). As a result
the full potential of the reinforcing fibres is not realised (see Section 2.8.5). In
recent years therefore, there have been a number of developments in reinforced